1. Field of the Invention
The present invention relates to a mobile communication device and a high-frequency composite unit included therein, and more particularly, to a mobile communication device capable of operating in a plurality of different mobile communication systems, and a high-frequency composite unit used in such systems.
2. Description of the Related Art
In the state of the art, a dual-band cellular phone device capable of operating in communication systems supporting a plurality of frequency bands, such as the Digital Cellular System (DCS) using the 1.8 GHz band and the Global System for Mobile communications (GSM) using the 900 MHz band, has been proposed as a mobile communication device for use in Europe.
FIG. 9 is a block diagram showing a portion of a typical dual-band cellular phone device including a combination of DCS using the 1.8 GHz band and GSM using the 900 MHz band, by way of example. The dual-band cellular phone device includes an antenna 1, a diplexer 2, and two communication systems, DCS and GSM.
The diplexer 2 transmits transmission signals from the two communication systems, DCS and GSM, to the antenna 1, and distributes reception signals received via the antenna 1 to the two communication systems of DCS and GSM. The DCS network includes a high-frequency switch 3a for switching signals between a transmitter Txd and a receiver Rxd, and a low-pass filter 4a and a directional coupler 5a, which are subsequent to the high-frequency switch 3a and are connected to the transmitter Txd. The GSM network includes a high-frequency switch 3b for switching signals between a transmitter Txg and a receiver Rxg, and a low-pass filter 4b and a directional coupler 5b, which are subsequent to the high-frequency switch 3a and are connected to the transmitter Txg. The low-pass filters 4a and 4b are located between the high-frequency switches 3a and 3b and the directional couplers 5a and 5b, respectively, such that harmonic distortion caused by transmission power amplifiers (not shown) incorporated in the transmitters Txd and Txg is removed. The directional couplers 5a and 5b extract portions of the transmission signals, and send the results to automatic gain control circuits (not shown) to maintain constant gains of the transmission signals.
The operation of the dual-band cellular phone device is described below. For DCS transmission, the high-frequency switch 3a turns on the transmitter Txd. A transmission signal sent from the transmitter Txd through the directional coupler 5a, the low-pass filter 4a, and the high-frequency switch 3a is selected by the diplexer 2, and is transmitted from the antenna 1. For DCS reception, on the other hand, a reception signal received by the antenna 1 is selected by the diplexer 2, and the high-frequency switch 3a turns on the receiver Rxd before the reception signal is passed to the receiver Rxd. For GSM transmission and reception, the same operations are performed.
However, the foregoing dual-band cellular phone device, which is one conventional mobile communication device, has a problem as described below. That is, a directional coupler for extracting portions of transmission signals and sending the results to an automatic gain control circuit is disposed in each of the DCS and GSM transmission paths, thus increasing the number of components required on a circuit substrate. This results in a dual-band cellular phone device (mobile communication device) having an increased size.
Another problem is that since an antenna, a diplexer, DCS and GSM high-frequency switches, DCS and GSM high-frequency filters (i.e., low-pass filters), and DCS and GSM directional couplers are mounted on a single circuit substrate in a discrete fashion, matching circuits which achieve matching, attenuation, or isolation characteristics are further required between the diplexer and the high-frequency switches, between the high-frequency switches and the high-frequency filters, and between the high-frequency filters and the directional couplers. Therefore, the number of components is further increased, thereby greatly increasing the required mounting area, and ultimately increasing the size of the circuit substrate. This results in a large dual-band cellular phone device (mobile communication device).